The present invention relates to a new type of plasma antenna for use in an information transmission system and, in particular, to a surface wave driven plasma antenna formed within a dielectric tube enabling furtive communications.
Presently, antennas based on a plasma discharge are known. U.S. Pat. No. 5,594,456 discloses a device whereby a pulsed antenna is utilised for the transmission and reception of signals in Ground Penetrating Radar and high speed data communication applications. However, this device requires metallic electrodes with associated wires and a radio-frequency decoupling device to drive the plasma antenna which limit its applicability as a communications device and more specifically as a furtive communications device.
A surface wave driven plasma is also known, as set out in the publication Burykin Yu I., Levitskiy S. M. and Martyneko V. G. (1975) Radio Eng. Electron. Phys. 20, 86. However this publication does not concern itself with developing the plasma as a communications device. It is not obvious in the slightest that the combination of the abovementioned prior art would produce the present invention.
Conventional conducting element antennas are also known and used widely. However, these antennae are not furtive due to their metallic components. Additionally, plasma antennas may be made flexible in the sense that the radiation pattern may be altered by changing the plasma density, or conversely maintaining the radiation pattern when the frequency is altered. These possibilities are not possible with simple metallic elements in conventional antennas.
This identifies a need for an improved type plasma antenna using a furtive means of operation and overcoming the problems inherent in the prior art.
In accordance with one aspect of the invention, there is provided a system for information transmission having a plasma antenna. The system may include an electrodeless plasma tube; and a power source effective to generate an electromagnetic field to cause ionization of material within the tube so as to form the antenna for one or both of either sending or receiving signals, wherein the electromagnetic field is applied to a portion of the tube.
The system may also include a terminal arranged about the tube at said portion for establishing the electromagnetic field upon application of power from the power source to induce surface wave ionization within the tube.
The use of surface wave ionisation provides a significant advantage over the antenna disclosed in U.S. Pat. No. 5,594,456 in that the plasma can be formed utilising only a single terminal and the metallic electrodes of the prior art may be dispensed with. This has particular advantage in stealth applications where metal componentry needs to be minimised to reduce a radar cross-section. Further, a single terminal may be used to both derive the plasma and generate a transmission signal which reduces component parts. Another specific advantage is that the antenna in tunable in the sense that the extent of surface wave ionisation can be controlled, allowing for dynamic control of the length and thereby operational frequency of the antenna. None of these advantages are contemplated or suggested in the prior art.
Preferably, the system comprises a furtive wireless communications device, said apparatus acting as either, or both, the transmitter and the receiver. By xe2x80x9cfurtivexe2x80x9d is meant that the antenna is only in existence and detectable when in operation. As soon as ionising power is terminated, the antenna ceases to exist.
Preferably, the system employs a means to use multiple frequencies simultaneously for the functions of plasma formation and maintenance, and signal transmission and reception.
Preferably, the plasma density and/or plasma dielectric properties is/are controllable by external means including, but not limited to, radio-frequency power supplied to said plasma excitation means, the frequency of said radio-frequency power, phase changes of the radio-frequency power, an applied magnetic field, the gas pressure or a gases partial pressure.
In another aspect, there is provided a method of communication, including providing an electrodeless plasma tube an establishing a plasma in the tube by surface wave ionisation to form a plasma antenna for one or both of either receipt or transmission of signals.
Preferably, the method includes controlling the plasma density and/or plasma dielectric properties by external means including, but not limited to, the radio-frequency power supplied to said plasma excitation means, the frequency of said radio-frequency power, phase changes of the radio-frequency power, an applied magnetic field, the gas pressure or a gases partial pressure.
Preferably, the method includes providing an array of plasma tubes, individual tubes being arranged and excited as to permit control of the overall radiation pattern arising from the array of antennae, the mutual coupling between individual antennae, frequency stepping of individual antennae, power loading of individual antennae, and the tuning of the array of antennae.